.content-article {
  position: relative;
  width: 100%;
  overflow: hidden;}
.content-article:before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  border-width: 16px 16px 0 0;
  border-style: solid;
  border-color: #dae2ea #889bae #dae2ea #889bae;
  background: #dae2ea;}

Resource: http://nicolasgallagher.com/pure-css-folded-corner-effect/